I am investigating this build_runner error when using Jaspr. After some digging I found a reproducible setup and think its a general build_runner bug. It boils down to the following:

I have two builders:

• Builder A reads in .a and generates .b (e.g. main.a -> main.b)
• Builder B reads in .b and generates .c (e.g. main.b -> main.c)

Builder A has some logic that it only conditionally generates .b files and that condition might change while watching the files (dart run build_runner watch).

When that happens, Builder A no longer outputs main.b. At that point I expect build_runner to also cleanup main.c since Builder B is also no longer executed since it doesn't have any input. However main.c still persists with now outdated contents.

To reproduce do the following steps:

1. Create a blank dart project.
2. Add build as a dep and build_runner as a dev dep.
3. Add the following files:

builders:
  a:
    import: "package:buildtest/builders.dart"
    builder_factories:
      - aBuilder
    build_extensions:
      .a:
        - .b
    auto_apply: all_packages
    build_to: source
    required_inputs: [".a"]
  b:
    import: "package:buildtest/builders.dart"
    builder_factories:
      - bBuilder
    build_extensions:
      .b:
        - .c
    auto_apply: all_packages
    build_to: source
    required_inputs: [".b"]

import 'dart:async';

import 'package:build/build.dart';

Builder aBuilder(BuilderOptions options) => ABuilder();
Builder bBuilder(BuilderOptions options) => BBuilder();

class ABuilder extends Builder {
  @override
  FutureOr<void> build(BuildStep buildStep) async {
    var content = await buildStep.readAsString(buildStep.inputId);
    if (content.contains('on')) {
      await buildStep.writeAsString(buildStep.inputId.changeExtension('.b'), content);
    }
  }

  @override
  Map<String, List<String>> get buildExtensions => {
    '.a': ['.b'],
  };
}

class BBuilder extends Builder {
  @override
  FutureOr<void> build(BuildStep buildStep) async {
    var content = await buildStep.readAsString(buildStep.inputId);
    await buildStep.writeAsString(buildStep.inputId.changeExtension('.c'), content);
  }

  @override
  Map<String, List<String>> get buildExtensions => {
    '.b': ['.c'],
  };
}

test on

4. Run dart run build_runner watch -v
5. See lib/test.b and lib/test.c being generated.
6. Change lib/test.a to test off
7. See lib/test.b correctly being removed, but lib/test.c wrongly being still there.

In the real-world case I experienced, BuilderA is a custom builder by Jaspr that generates web/<filename>.dart files that should be compiled to js. BuilderB is ddc_modules from package:build_web_compilers that outputs .ddc.module files for each .dart file. The error occurs in the ddc builder that for each .ddc.module tries to read the respective .dart file, which doesn't exist for an outdated module.

You can reproduce the error by creating a new Jaspr project (jaspr create testapp), starting build_runner watch and removing the @client annotation from e.g. lib/pages/about.dart.

dart --version: Dart SDK version: 3.7.0 (stable) (Wed Feb 5 04:53:58 2025 -0800) on "macos_arm64"
build_runner: ^2.4.15 (latest)
build: ^2.4.2 (latest)